1. Reasonable process design
In the design, the steam is placed on the tube side to avoid high-speed gas flowing through the shell side. When there is a large flow of medium in the shell side, multiple shell side inlets can be designed to buffer the pressure. In addition, anti scour plates should be set to reduce the erosion and corrosion of equipment caused by high-speed fluid.
In order to avoid the retention of residual liquid and sediment, double-sided butt welding and continuous welding shall be used as far as possible to avoid lap welding and spot welding. In the welding process, according to the practical experience, the stress causing stress corrosion cracking is mainly residual stress, which is mainly composed of the internal stress caused by cold working and welding.
The heat treatment of cold working parts and welding parts is helpful to eliminate the residual stress and prevent the stress corrosion. Stress annealing heat treatment is often used to eliminate residual stress or other methods to eliminate residual stress, such as hydrostatic test, vibration aging and hammering.
In addition, the tube bundle shall be lifted with nylon belt to ensure that the metal surface is flat, free of scratches and can enter the shell smoothly.
2 corrosion resistant materials
The use of corrosion-resistant materials (such as binocular stainless steel, Hastelloy, titanium, titanium alloy, copper, etc.), which have strong corrosion resistance, can improve the service life of heat exchanger, but these high corrosion-resistant materials are expensive, high manufacturing cost, large cost of one-time investment, generally difficult to accept by enterprises and difficult to promote.
3. Electrochemical protection method
The electrochemical protection method can not only prevent the stress corrosion fracture, but also stop the propagation of the crack even if the protection parameters are properly selected. Methods of sacrificial cathodic / anodic protection or surface spraying of corrosion-resistant metals may be used.
The anode on the metal surface is changed into a cathode and protected by an external DC power supply. This method consumes a lot of electricity, costs a lot and is rarely used.
Anode protection method:
The protected equipment is connected with the anode of the external power supply to form a passivation film on the metal surface, so as to achieve protection. The cost of carbon steel heat exchanger is low, but its corrosion resistance is poor.
The service life of the heat exchanger can be improved by adopting the sacrificial anode protection technology, but the protection effect of this technology is only in the limited length of the pipe inlet, and it is difficult to realize the cathodic protection in the depth of the pipe, so the application of the sacrificial anode protection method in the heat exchanger has been greatly limited.
4. Method of adding inhibitor
In corrosive medium, a small amount of some substances are added, and these substances can greatly reduce or even stop the corrosion of metals. This kind of substances are called corrosion inhibitors. Fig. 6 is the comparison before and after the use of corrosion inhibitor. The addition of corrosion inhibitor shall not affect the production process and product quality.
Stress corrosion can be controlled by removing dissolved oxygen and oxidants from the medium. Reducing the mass concentration of Cl - in the medium and strictly controlling the mass concentration of sulfur in the medium are also effective measures to control stress corrosion.
5 anti corrosion coating method
In order to avoid the direct contact between the metal surface and the corrosive medium, a layer of corrosion-resistant coating is covered on the metal surface by a certain coating method.
This technology is economical and effective, and is used to prevent gas medium corrosion. Most of the coatings used are organic polymer mixture solutions. At present, people are gradually developing towards oil and solvent resistant coatings, high temperature coatings, heavy-duty anticorrosive coatings and coatings for special environment.
6 operation of heat exchanger
During the start-up of the heat exchanger, the cold fluid is filled into the container, the inlet is closed, and then the hot fluid is injected slowly, so as to minimize the thermal expansion difference between the pipe and the shell formed by the introduction of the fluid.
After shutdown, use dry compressed air to remove all the fluid in the heat exchanger, so as to reduce the stress to a small level and avoid stress corrosion. During the start-up process, the upper and lower water valves shall be kept in full open state to avoid the slow flow rate, and the impurities in the medium precipitate on the pipe surface to cause scaling and corrosion.